Over the years, several different approaches have evolved for developing, printing and duplicating visible impressions on graphic media, such as paper or the like. A traditional approach to printing and duplicating typically uses ink or ribbons. Other prior approaches commonly use recording paper for copying and duplicating visible images. Recording paper has proved to be advantageous because it does not require the use of ink or ribbons and is generally reliable over a large range of conditions.
One type of recording paper commonly used is photographic paper which utilizes photographic processes and techniques to reproduce illustrations and images. Typically, light causes a chemical reaction to photosensitize a chemical on the photographic paper. This is followed by another chemical process which introduces a marking reagent to make the sensitized area visible. The use of photographic paper has some disadvantages. For example, it requires a two step process and complete darkness during the development process. In addition, it takes several minutes for the image to be developed.
Another type of recording paper is direct developing paper which reduces the two step process to a single chemical reaction. This reaction is caused by either electrical, magnetic or thermal energy. The chemical reaction renders the sensitized area visible.
Thermal recording paper, which is widely used in facsimile equipment and printers, was originally developed for use in instrument recorders used in airplanes and the like. In instrument recorders an ink pen was replaced with a heated wire which would write on the surface of the paper to form an image. Typically, the wire to be heated was mounted to a mechanical arm which would move across the paper. Subsequently, in order to replace the mechanical arm in these recorders, an array of hot wire pens was created. In such an arrangement, as the strip of thermal paper was moved past the array, the appropriate hot wire element would mark the paper to form an image. These arrays also form the print head in alphanumeric and graphical printers. Facsimile machines currently utilize hot wire elements to record images. The hot wire element operates in accordance with Joule's model of electric heating whereby resistance in a conductor produces heat. This heat causes a chemical reaction in a coating on the paper, which produces a visible change. This is not always desireable because the resolution of the image is often determined by the size of the heating element.
There are several different approaches to thermographically reproducing images. One approach creates special embossed effects in printing, such as stationary, invitations, greeting cards and paper decoration. To form a raised surface resembling die engraving without using costly engraving dies, special non-drying inks are applied, either by letter press or offset, and the wet inks are dusted with a powdered compound. After the excess powder on the non-printing areas is removed by suction, the sheet passes under a heater which fuses the ink and powder compound. The printing swells or raises in relief to produce a pleasing engraved effect.
Another approach is a reflex process, known as a dual spectrum process, which utilizes an original copy superimposed with a translucent sheet having a photosensitive coating. The photosensitive coating is not apparent to the unaided eye. Exposure to a brilliant light for several minutes causes the light to transmit through the translucent sheet, reflect off the original and alter the nature of the photosensitive coating. An opaque sheet having an infrared sensitive chemical coating is then positioned in contact with the translucent sheet. A second exposure, this time to infrared radiation, causes a chemical reaction in the infrared coating which reproduces the images on the opaque sheet. This process takes several minutes.
Another thermographic approach, which is also a reflex process, utilizes an original superimposed first with a transfer sheet and then with a recording or copy sheet which is a transparent or translucent paper or plastic sheet. The recording sheet has an adhesive layer which is positioned on the transfer sheet. Direct exposure to infrared radiation softens the adhesive layer on the recording sheet. The radiation transmitted in large part through the recording sheet and completely by the transfer sheet, is absorbed by the images on the original sheet. The absorbed radiation on the original generates a heat pattern corresponding to the shape of the original images and the heat pattern is conducted back to the transfer sheet causing portions of the transfer layer to melt. The melted portions of the transfer layer are absorbed into the areas of the adhesive layer in contact with the transfer layer to form imaged areas in the adhesive layer which are legible as direct reading images.
These prior approaches utilize transfer sheets, relatively large and undesirable amounts of infrared radiation and several minutes to form the images. In addition, all the prior approaches are relatively complex and expensive.
A need thus exists for an apparatus for developing, printing and duplicating graphic media which provides a direct and simple process by eliminating the use of transfer sheets, multiple chemical reactions and infrared radiation sources emitting hazardous amounts of infrared radiation. It would also be desirable to have a low cost apparatus and less expensive process.